The present invention relates generally to femoral sizing guide. In particular, embodiments of the present invention relate to femoral sizing guides for accurately positioning a cutting block on a resected distal portion of a femur in order to locate cutting planes for preparing the end of the femur to receive a prosthetic implant. The present invention also relates to methods of using femoral sizing guides.
During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure, or joint arthroplasty, may involve the use of a prosthesis which is implanted into one of the patient's bones.
During performance of a joint replacement procedure, it is generally important to provide the orthopedic surgeon with a certain degree of flexibility in the selection of the correct size of prosthesis. In particular, the anatomy of the bone into which the prosthesis is to be implanted may vary somewhat from patient to patient. In order to implant a prosthetic joint, it is commonly necessary to prepare the bone to receive the prosthesis. For a prosthetic knee joint, both the distal femur and the proximal tibia may need to be accurately resected to shape the ends of the bones to receive the implants. For preparing the distal femur as a first preparatory step, a transverse surface is formed at the distal end of the femur by performing a first resection. This resection may be located using separate instruments, not described in detail here.
Femoral knee prostheses are typically provided in a range of standard sizes. Once selected, the femoral knee prosthesis must be located and oriented to provide appropriate rotational alignment. Correct selection of the size of prosthesis and correct positioning of the prosthesis relative to the natural bone is essential to ensure natural movement of the assembled joint. In particular, the implant must be positioned to provide an appropriate gap between the femur and the tibia when the knee is in extension and in flexion, and to ensure that the surrounding tissues are correctly balanced. It is known to set the rotation of the femoral prosthesis relative to Whiteside's line, which extends from the intercondylar notch to the patella groove. Alternatively, the rotation of the femoral prosthesis may be set relative to the transepicondylar axis, which connects the high points of the epicondyles. Further anatomical reference marks may also be used.
It is known to use a femoral sizing guide mounted on the resected distal femur surface to measure the size of the distal femur, and in particular the size of the condyles and to specify the required locations of guiding apertures into the femur to secure an appropriate cutting block to the femur to prepare the femur to receive the prosthesis. A surgical instrument set for performing a knee replacement procedure including a femoral sizing guide is marketed by DePuy Orthopedics, Inc. under the trade mark Sigma High Performance Instruments. The Sigma HP instrument set allows surgeons to perform total knee arthroplasty procedures under most surgical approaches.
The Sigma HP femoral sizing guide is used to determine the correct size for the femoral prosthesis and to position guide pins to support a cutting block on the distal end of the femur. A body portion of the femoral sizing guide is seated upon the resected femoral surface. Posterior feet extend from the body underneath the posterior condyles. The femoral sizing guide is correctly located when the posterior condylar surfaces rest upon the feet with Whiteside's line extending generally centrally through the sizing guide. When the feet are correctly located the body is can be secured to the bone with pins which extend into the bone through fixed position pin holes. A stylus is coupled to the body such that it can be raised and lowered and locked in position. The stylus tip extends over the anterior cortex of the distal femur. The stylus can also rotate about an axis extending generally parallel to the resected surface and can slide through the coupling to the body along the anterior cortex generally parallel to the longitudinal axis of the femur. The tip of the stylus is positioned upon the anterior cortex of the femur at the intended exit point of the anterior cut for the femoral prosthesis. The height of the stylus above the condylar feet can be read off a scale upon the body and indicates the size of the required prosthesis. A scale on the stylus indicates the exit point on the anterior cortex for each size of prosthesis. The chosen size of the femoral prosthesis determines the size of cutting block to be coupled to the distal transverse surface of the femur. The stylus position sliding through the sizing guide and the height of the stylus above the feet are set to the same value on the respective scales (corresponding to the size of the femoral prosthesis). When the tip of the stylus just contacts the anterior cortex as the stylus sweeps across the anterior cortex, the anterior part of the corresponding size of femoral prosthesis will terminate at the anterior surface of the bone without leaving a notch or an overhang.